It has been demonstrated that the initiation and cessation of DNA synthesis in response to anti-immunoglobulin occurs at the same surface membrane site and that the on signal is independent of internalization of anti-immunoglobulin. The lipid A moiety of lipopolysaccharide induced bone marrow cells to secrete a lymphokine that is mitogenic for splenic B cells. A differential effect of vanadate on the mitogenic response induced by T and B cell mitogens was demonstrated. Vanadate may provide a useful probe to study the mechanism of T cell activation.